The invention relates to the control of a gas turbine, in particular, but not exclusively of a gas turbine for an aeronautical engine, more particularly a turbo-jet engine.
In a turbo-jet engine, several control loops are provided, more particularly:                a main control loop for slaving the speed of the turbo-jet engine on a speed setpoint depending on the desired thrust, by action on the flow rate of fuel fed to the combustion chamber of the gas turbine, and        local control loops for slaving the position of portions of variable geometry.        
In a turbo-jet engine with several spools, for example with a low pressure (LP) spool (compressor and turbine) and a high pressure (HP) spool, the magnitude servo-controlled by the main control loop may be the speed of rotation NLP of the shaft connecting the LP turbine to the LP compressor. Another magnitude may be used, in particular the engine pressure ratio (or EPR), i.e. the ratio between the pressure at the inlet of the LP compressor (or fan) and the pressure at the outlet of the LP compressor (or fan).
The portions of variable geometry, designated also hereafter by “variable geometries” have positions slaved on setpoint values depending on the speed of the turbo-jet engine, which may be reduced, or during transient phases (switching from one speed to another). Well known examples of variable geometries are the assemblies of compressor stator vanes with a variable setting angle, or VSV (Variable Stator Vanes), valves for picking up air for discharging the compressor, or VBV (Variable Bleed Valves) which are controlled during the whole operating time of the turbo-jet engine in flight, or further valves for picking up air for transient discharge of the compressor or TBV (Transient Bleed Valves) which are controlled during particular flight phases.
In order to ensure sound operation of the turbo-jet engine, in particular in order to avoid stallings of the compressor, extinctions of combustion or overspeeds, thresholds are introduced into the main control loop, which limit the increase or decrease of the fuel flow rate during a requested change of speed for the turbo-jet engine. These thresholds are computed depending on the estimated thermodynamic state of the turbo-jet engine at the instant of the change in speed. Now, in some circumstances, and in particular during successive transitions between speeds, a significant difference may exist between the estimated thermodynamic state of the turbo-jet engine and its actual state, which imposes that an additional margin be taken on the computed thresholds or on the dimensioning of the compressor.